TGR23, a G protein-coupled receptor, is characterized in having an amino acid sequence represented by SEQ ID NO: 1 or SEQ ID NO: 2 of the sequence listing. The TGR23 and its ligand (TGR23 ligand) are reported in WO 02/31145 (Patent Document 1) and WO 03/025179 (Patent Document 2), and when TGR23 responds to the ligand, the dietary behavior is suppressed, and tumors proliferate (WO 03/025179). In this regard, an antagonist of TGR23 is useful as a low toxic and safe medicine, such as a prophylactic and/or therapeutic agent for cancers (e.g., large intestine cancer, colon cancer, rectal cancer, breast cancer, lung cancer, non-small cell lung cancer, prostate cancer, esophageal cancer, stomach cancer, liver cancer, bile duct cancer, spleen cancer, renal cancer, urinary bladder cancer, uterine cancer, ovarian cancer, cervical cancer, testicular cancer, thyroid cancer, pancreatic cancer, brain tumor, blood tumor and the like), a feeding (appetite) promoting agent, a prophylactic and/or therapeutic agent for anorexia, an apoptosis inducing agent, and the like.
Meanwhile, the following bicyclic piperazine compounds are known.
(1) The following compounds are described in Synthesis, p. 833 (1996) (Non-Patent Document 1) as 2,6-disubstituted piperazine derivatives synthesized by an asymmetrical synthesis method, but no description is given on the action and use of the compounds.

(2) The following compound is described in WO 97/19089 (Patent Document 3) as an intermediate of an oxazolidinone derivative having antibacterial action.

(3) The following compound is described in WO 98/58947 (Patent Document 4) as an intermediate of a dipeptide derivative having a growth hormone secretagogue action.

(4) The following compound is described in J. Org. Chem., Vol. 67, p. 3866 (2002) (Non-Patent Document 2) as a compound relevant to a monocyclic oxopiperazine derivative having affinity to a CCK receptor.

(5) The following compound is described in WO 03/30907 (Patent Document 5) as an exemplary compound useful as a prophylactic and/or therapeutic agent for inflammation, cell adhesion-mediated diseases and central nervous diseases.

[Patent Document 1] WO 02/31145
[Patent Document 2] WO 03/025179
[Patent Document 3] WO 97/19089
[Patent Document 4] WO 98/58947
[Patent Document 5] WO 03/30907
[Non-Patent Document 1] Synthesis, p. 833 (1996)
[Non-Patent Document 2] J. Org. Chem., Vol. 67, p. 3866 (2002)